The invention relates to formation of ferrite layers having application in micro-electronic device manufacturing, and more particularly to spin-spray processes for ferrite film fabrication.
Conventional ferrite film fabrication techniques include sputtering, vacuum evaporation, molecular beam epitaxy and liquid phase epitaxy. While the correct chemistry can be obtained at close to ambient temperatures, formation of the desired crystaline phases using these techniques requires temperatures as high as 1000xc2x0 C. Some substrates, such as GaAs and plastics, may not withstand such high temperatures. Silicon-based integrated circuits often do not permit temperatures above 400xc2x0 C. for processes applied after the devices are fully formed. Ferrite films have also been formed by spraying an aqueous oxidizing agent solution on a substrate and simultaneously supplying an aqueous reaction solution containing ferrous ions to effect a crystallization reaction on the substrate. Such processes have been performed at temperatures below 100xc2x0 C. These spray techniques, however, generally do not provide adequate in-plane anisotropy.
In addition to these temperature issues, bulk ferrites typically have ferromagnetic resonance frequencies in the range of 100 to 300 MHz, limiting their use to devices operating below these relatively low frequencies. Beyond the ferromagnetic resonance frequency, the material can no longer respond to a magnetic field. Accordingly, above this frequency an inductive device can no longer act as an inductor.
It is known that increased anisotropy in the film plane may increase ferromagnetic resonance frequency, thereby allowing operation at higher frequencies. Uniaxial anisotropy has been generated by crystallizing sputtered or vapor deposited amorphous films. The crystallization is typically promoted by heat treating the film in a magnetic field. These methods, however, usually require temperatures of between 350xc2x0 C. and 650xc2x0 C., thereby restricting their use to substrates and devices not adversely affected by temperatures in this range. Consequently, applications of such techniques are limited.
Accordingly, there is a need for a low temperature ferrite formation process, that preferably produces a highly anisotropic structure.
Embodiments of the invention provide a ferrite layer formation process that may be performed at a lower temperature than conventional ferrite formation processes. The formation process may produce highly anisotropic structures.
According to embodiments of the invention, a ferrite layer is deposited on a substrate while the substrate is exposed to a magnetic field. The process is particularly applicable to spin coating processes. In an illustrative embodiment of the invention the magnetic field is in the plane of the substrate. An intermediate layer may be positioned between the substrate and the ferrite to promote bonding of the ferrite to the substrate. Advantageously, the process may be performed at temperatures less than 300xc2x0 C., and even more advantageously at temperatures below 100xc2x0 C.
In-plane uniaxial ferrite film anisotropy may be achieved by embodiments of the invention in the range of about 1000 dyn-cm/cm3 to about 2xc3x97106 dyn-cm/cm3.